This invention is directed to a method and system which removes several regulated, toxic mineral and organic precursor substances from drinking water or wastewater by causing them to be adsorbed and absorbed onto sulfur-activated sponge iron particles, which may be referred to as "sulfur-modified iron". Specifically, the method and system removes from water trace amounts of (1) dissolved, colloidal, and particulate arsenic, selenium, and lead; and (2) naturally-occurring organic compounds (TOC, total organic carbon in the water), which, when oxidized, form "disinfection byproducts"; and (3) other potentially harmful minerals.
The subject matter of this invention is related to some extent to that of U.S. Pat. Nos. 4,940,549 and 5,200,082, which are directed to removal of selenium from agricultural drain water and from refinery effluents and other industrial waste waters.
The process of the present invention is particularly concerned with removal of arsenic and other toxic metals, and lowering the level of TOC from drinking water, using some of the same steps which were found efficacious in removal of selenium in the above referenced patents.
Recent scientific investigations by the United States Environmental Protection Agency (EPA) and others have suggested that arsenic in drinking water causes cancer in humans; that no quantity of arsenic in drinking water is a safe quantity; that arsenic may not be an essential human nutrient as previously reported by science; and that the cancer risk from ingesting arsenic at the currently permitted level in drinking water may equal that caused by smoking cigarettes. The EPA currently is negotiating the content of the regulations for arsenic in drinking water and it projects that the new limitation will be between 2 and 5 micrograms per liter (.mu.g/L), most probably 2 .mu.g/L.
Many existing water systems will have to treat to reduce the concentration of arsenic. Three examples follow:
(a) The California State Water Project which has naturally-occurring arsenic in the amount of 2 to 6 .mu.g/L provides water for several million families;
(b) The City of Hanford, Calif. gets its drinking water for more than 20,000 people from ground water wells. Some of the wells have arsenic concentrations near the current limit of 50 .mu.g/L;
(c) The Kern Water Bank is a project constructed to store water in underground reservoirs in times of plenty for use when water is not available. Some of the Water Bank wells have naturally-occurring arsenic. In a few, the arsenic concentration approaches 200 .mu.g/l.
Water providers have an immediate need for an economical, safe method to remove arsenic from drinking water. Current methods, systems, and technologies have either proved to be uneconomical or ineffective to meet the proposed 2 .mu.g/L standard. The test for economical water service is as follows: Is the cost of water less than two percent of the gross income of a family at the poverty level?
Existing technologies for removal of arsenic include the following: (a) adsorption onto activated alumina within a fixed bed contactor; (b) complexing arsenic with hydrous metallic floc, primarily aluminum and iron hydroxides or oxyhydroxides, in conventional water treatment plants; (c) sieving the metal from water by membrane technologies such as reverse osmosis; and (d) electro-dynamic processes such as electrodialysis. The present invention described below can exhibit a cost advantage of 5 to 15 times compared to these prior methods.
Also, since the middle 1970s, the EPA has warned that certain classes of byproducts formed by oxidizing naturally-occurring organic acids during disinfection are potentially carcinogenic. These compounds are regulated as "disinfection byproducts" (DBPs) to limit consumption. There are many DBP compounds of interest to EPA. Not all have been fully described or investigated with respect to their potential effects on human health or the frequency of their occurrence in domestic water systems. In addition, the epidemiological impact of DBPs is uncertain. Hence, there is a concerted effort of national scope to develop data on the formation of DBPs and to better define their potential impact on human health.
The EPA is currently requiring more water systems to disinfect their water to limit the occurrence of waterborne disease, while at the same time the EPA is seeking to reduce the impacts of DBPs on human health; these can be conflicting purposes. Most water systems disinfect their water. Thus, many water systems will have to initiate or modify water treatment systems to reduce DBPs to meet proposed trihalomethane limitations which may range from 40 to 60 .mu.g/L. Other DBPs, such as haloacetic acids and several bromine compounds, will be subject to numerical concentration limits. Conventional treatment systems for surface water sources may meet many of the proposed standards if influent precursors, i.e. TOCs, can be limited to 4 mg/L prior to disinfection with chlorine.
Thus, across the United States, water providers have an immediate need for an economical, safe method to reduce the occurrence of DBPs in drinking water. Current methods, systems, and technologies have either proved to be uneconomical or ineffective to meet the proposed 40 to 60 .mu.g/L standard. The EPA has been proposing stringent limitations on the class of DBPs known as trihalomethanes, cited above, since 1975. Water providers have often failed to meet the most basic requirement that total trihalomethanes be less than 100 .mu.g/L.
As noted above, the primary strategy presently used to reduce DBPs is to control precursor chemicals early in the water treatment process so that smaller quantities of disinfection byproducts form during disinfection. Current alternative strategies include use of non-conventional disinfectants, treatment of the water to reduce formation of DBPs (in the presence of conventional disinfectants), and removal of DBPs after formation. Existing technologies for reducing the concentration of DBPs include the following: (a) adsorption of DBPs or precursors onto granular activated carbon within a fixed bed contactor or adsorption onto powdered activated carbon during various stages of the treatment process; (b) complexing DBPs or precursors with hydrous metallic floe, primarily aluminum and iron hydroxides, in conventional water treatment plants after adjusting the pH of influent water; (c) sieving the relatively-larger organic molecules from water by membrane technologies such as ultrafiltration; and (d) electro-dynamic processes such as electrodialysis.
It is an important object of the present invention to efficiently and very economically remove arsenic, TOCs, and other contaminant metals from drinking water. Procedures described can reduce cost of removing these contaminants by a factor of five to fifteen, particularly as compared to reverse osmosis or nanofiltration. It is a further object of the present invention to provide a method and system for removing arsenic, TOCs, and other contaminant metals which may be introduced into existing water treatment facilities. These and other objects of the invention will be apparent to those skilled in the art from the detailed description of the invention contained herein and from the accompanying drawings.